Sailboards are rigged with a sail extending from a mast mounted on the flat board or platform hull, with a universal joint connection between the platform and mast allowing trim manipulation of the sail by a rider standing on the platform grasping a wishbone shaped boom attached to the mast and extending on either side of the sail. The sail is tensioned with an outhaul tackle attached to the clew of the sail and the outer end of the boom.
Sailboard sails are generally triangular in outline. The sail generally has a mast sleeve which is closed at the head and open at the foot, and is rigged by inserting the mast into the sleeve. Downhaul tackle, just above the universal joint, secures the tack and allows adjustment of luff tension. The wishbone boom attaches to the mast at mid-height through a cutout in the sleeve. The rider controls the board's direction by tilting the boom forward or aft and its velocity by sheeting the sail in or out. He balances the aerodynamic forces of the sail against the hydrodynamic ones of the platform hull and fin by movement of the boom, positioning his weight and controlling the thrust line of his body.
In spite of the apparent simplicity of the sailboard's rig, the lack of external tackle or standing rigging makes it one of the purest expressions of applied aerodynamics. Since the rider's total reactive force is limited, high performance sails must have a good lift/drag ratio, a low center of effort and predictable handling characteristics over a wide range of wind speeds. Predictable handling results from the maintenance of a stable sail cross-section in all wind speeds.
Presently, the two most distinct categories of sailboard sails are the "RAF" (rotating asymmetrical airfoil) and the camber induced sail. The RAF was the earlier development. In an RAF, a carefully controlled mast sleeve diameter allows a tangential positioning of the sleeve and battens on the leeward side of the mast. This greatly increases the sail's efficiency by maintaining better flow attachment on the leeward surface of the foil. Wind tunnel tests have shown that the tangential position of the sail relative to the mast produce substantial lift/drag improvements over a conventional inline arrangement of mast and sail.
The term "sail rotation" is used to describe the motion of battens and mast sleeve around the mast's centerline. In order to maintain a preload in the sailcloth when the sail is rigged, the sail's luff curve is cut to match the mast bend for a similar loading. The sail is laid up of several panels as if covering an airfoil form. The abutting edges of the panels are curved convexly to fit this spherical surface. This panel curvature used to control the sail's three dimensional shape is described in sailmaking parlance as "broad seaming".
In spite of the RAF's improved efficiency it had two major problems: maintaining batten tangency and rotational stiffness.
Batten tangency problems result either from a mismatch of mast and mast sleeve diameter or that the mast's bending curve was not matched to the sail design. As a result of a misfit, the sleeve might be too tight, not allowing complete rotation of the battens to the mast tangency point. Or it could be too loose, allowing over rotation and permitting the battens to project forward of the tangency point. Over rotation also resulted from trying to rig the sail on too stiff a mast. In this case the excess luff round pushes the battens beyond the mast when downhaul tension is applied. Over rotation forms a ridge, interrupting smooth flow onto the sail. Either over or under rotation significantly reduces the sails aerodynamic efficiency.
The ideal sailboard sail has high rotational stiffness, allowing the sail to be depowered by reducing its angle of attack. If the cross-section is stable, this allows reduction of the lift coefficient without changing the moment characteristics of the foil. However, if the rotational stiffness of the joint formed by the mast sleeve and the sail body is low, the sail will easily rotate to an inline position. The three dimensional surface of the sail is only stable at the mast tangency points. Movement to an inline position will destabilize the cross-section. So, as the rider tries to depower an RAF, or the sail tension increases in a strong gust, the sail derotates, permitting the leading edge to cave in and the draft to move aft, generating a large pitching moment.
Unlike a boat, with a fixed mast and stays, the sail forces of a windsurfer must all be resisted by the rider. Straight vector resolution isn't a problem but a moment is. Most air-foils develop some negative overturning moment (pitching moment). In general, a forward position of the maximum camber, and a straight foil section aft produce the lowest pitching moment. In an RAF the only forces available to resist rearward movement of the draft are its three dimensional panel shaping and the rigidity of the battens.
Camber reduction also reduces the lift coefficient. As the windspeed increases, the only viable option with an RAF is to increase the outhaul tension reducing the camber. Unfortunately, the resulting foils are difficult to control, since flatter sections have very abrupt stall characteristics.
Some improvement in draft control has been achieved by the use of lightweight tapered battens. Under compressive load, they bend in their forward third and are extremely stiff toward the leech. The larger leading edge curvature generated by these battens also reduces the severity of the RAF sleeve tangency problem but still cannot increase sleeve tension or rotational stiffness. So, in spite of these improvements, it has been clear that greater rotational stiffness, higher fabric tension and better draft stability are required to achieve predictable performance at higher wind speeds.
To address the RAF's deficiencies a new class of devices has been developed, here referred to generically as "camber inducers". These devices attempt to solve the aerodynamic streamlining, batten curvature and luff fabric tension problems simultaneously.
In an RAF, the end of the batten pockets defines the furthest forward point of the tensile load imparted to the sail. Since the batten does not thrust against the mast, the only tension in the mast sleeve is the vertical transmission of downhaul tension and a horizontal component derived from outhaul tension and sail loading. Camber inducers react the compressive load of the battens through an intermediate member to the mast. Although there are differences in their construction the operational characteristics of all of the camber inducers allows simultaneous tensioning of both the mast sleeve and sail body fabric all the way to the front of the mast. With this arrangement of force resolution, the fabric tension, including sleeve tension, is controlled by batten compression. The uniformity of fabric tension in the sail body is determined by the accuracy of its surface development and the local curvature of the battens. Draft is controlled by outhaul tension and the downhaul tension controls the curvature of the leading edge. Inducers also define the sail to mast tangency condition and directly increases bi-stable, rotational stiffness as a function of batten compression.
The camber inducers described in Nishimura U.S. Pat. No. 4,625,671, Magnan U.S. Pat. No. 4,686,921, Magnan, U.S. Pat. No. 4,708,079 and Magnan U.S. Pat. No. 4,856,447, are all mechanically equivalent to end loaded, loose jointed, toggle linkages. The differences between them are in the details of the hinge arrangement, mast sleeve support surfaces and batten retention within the inducer. Belvedere, U.S. Pat. Nos. 4,649,848 and 4,733,624 are actually a conformal fairing, but do allow the compressive load of the batten to tension both the sleeve and body fabric of the sail by reacting the thrust load, via the anchor strap to the mast centerline.
Split battens have been developed which react the compressive load of the batten to the mast via a stirrup.
All of the camber inducers above are described as couplings, rotationally attached to the mast, and are, therefore, constrained to operate in the plane of the linkage they define, at right angles to the mast.
While camber inducers are effective in improving mast sleeve tension, rotational stiffness and defining a stable cross-section, they do have some manufacturing and usage problems. The most obvious are rigging complexity, sleeve flooding, and manufacturing cost. A more subtle one is excessive rotational stiffness if the batten load is too great or too eccentric within the inducer body.
One of the great advantages of a sailboard over other sailing craft is its rigging simplicity, allowing a sailor to make an instant decision to get on the water if the conditions are good. Since conditions change rapidly sailors frequently use more than one sail size in a given day. Any sail that takes too long to rig cuts into valuable and limited sailing time. The time consuming mast, batten and inducer alignment necessary with most internal camber inducer systems has given rise to some simplified hybrids and has also kept the RAF alive.
The mast sleeve must be designed with openings to allow insertion of the inducers. These openings can vary from mere slits to complex zippers closures. However, in all cases they add to manufacturing cost, and the multiple openings are also difficult to seal leading to sleeve flooding.
A mechanical design constraint of all of the camber inducers cited above, which operate as planar linkages, is that they must act at right angles to the mast. This isn't the optimum aerodynamic angle for the thirty to thirty-five degree sweep angles of most windsurfing rigs. With the mast raked at these angles, the battens create a series of turbulence inducing ridges along the surface of the sail that promote early flow separation. The current trend toward stiffer and lighter tubular batten sections aft, increases the height and disruptive capability of these ridges. When applied to a typical, highly curved windsurfer mast, the right angle limitation of these devices usually confines their use to the three mid-height battens. The mast head and foot have simple RAF type battens that are hoped to be influenced by the nearby camber induced ones.
The goal of all the devices described above is the definition of stable cross-sectional profile combined with high chamber. Important operational features of devices used to accomplish it are that they should be as light as possible to facilitate handling, simple to rig and robust enough to take continuing abuse without failure.